The unique properties of titanium dioxide (TiO2) nanotubes have resulted in their use in several emerging advanced applications, such as their use as water photoelectrolysis catalysts (i.e., for hydrogen generation), photovoltaic components in dye-sensitized solar cells, and as hydrogen gas sensors. There is ongoing interest in using TiO2 nanotubes for numerous other purposes, including as electronic components, microfluidic devices, nanofiltration devices, drug delivery devices, photocatalytic devices, and tissue engineering components.
TiO2 nanotubes have been prepared by several methods, including electrochemical oxidation (i.e., anodization), hydrothermal synthesis, and template-assisted synthesis. The anodization method typically produces the most highly ordered nanotube structure and most promising photovoltaic properties, as described in, for example, C. A. Grimes, J. Mater. Chem., 17, 1451-1457 (2007). The anodization process is also favored due to its general simplicity and high controllability. The anodization process generally involves anodizing titanium in an electrolytic solution containing water or a polar organic solvent (e.g., ethylene glycol, formamide, N-methylformamide, or dimethylsulfoxide) having dissolved therein a fluoride-containing electrolyte, such as HF, KF, NaF, NH4F, or a tetraalkylammonium fluoride (e.g., Bu4NF).
However, several difficulties remain in anodization processes of the art. For example, the anodization processes of the art are limited in their ability to produce TiO2 nanotubes having improved photovoltaic (PV) properties, particularly those in which an improved PV property results from a finer outer tube diameter (e.g., of or less than 45 nm) and/or higher aspect ratio, e.g., a length-to-diameter aspect ratio of at least 100. In addition, the electrolyte solutions used in anodization processes of the art are generally limited in their attainable electrical conductivity, thereby imposing a high energy usage during anodization. Furthermore, the polar organic solvents of the art (which are gaining in popularity over aqueous solutions) tend to possess one or more unfavorable properties, such as high volatility, flammability, and/or toxicity.
Accordingly, there is a need for a process that can produce TiO2 nanotubes having improved photovoltaic properties, particularly those having finer outer tube diameters (e.g., of or less than 45 nm) and/or higher aspect ratios, e.g., a length-to-diameter aspect ratio of at least 100. There would be a further advantage if such a process was capable of growing the TiO2 nanotubes at substantially high growth rates (e.g., at least 0.2 μm/hr). There is also a need in the art for a process that can produce TiO2 nanotubes with less energy usage, preferably by virtue of an increased electrical conductivity of the anodization medium. There is an additional need in the art for such a process which also does not require solvents that are volatile, flammable, toxic, or which present an environmental liability.